Cruel? Nitrogen asphyxiation is probably one of the most painless, gentle ways to go.

Your trigger that you can’t breathe is a buildup of carbon dioxide. But as you can still exhale, you feel no panic. You just slowly drift unconscious and die. I’d take it over most causes of death.

There's a BBC documentary about it, I think this one:

How to Kill a Human Being

It's been a long time since I watched it, but I think the inert gas route is very pleasant. He even gets slightly high/happy from it.

Key takeaways:

• there are surprisingly easy ways to kill people humanely.
• many in the US doesn't want to kill prisoners humanely, they want it to hurt and be a punishment, not die in a euphoric high

edit: found it:

https://www.documentarytube.com/videos/how-to-kill-a-human-being-2/

Rendered unconcious within 15 seconds, dead within a minute.

In testing pigs would happily stick their heads in a space with pure nitrogen and munch on apples till they lost consciousness, fell over, then stick their heads back in the space with nitrogen to eat some more apples.